The subject matter relates generally to medical devices and more particularly to classification of sensed cardiac complexes.
Effective, efficient ventricular pumping action depends on proper cardiac function. Proper cardiac function, in turn, relies on the synchronized contractions of the heart at regular intervals. When normal cardiac rhythm is initiated at the sinoatrial node, the heart is said to be in sinus rhythm. However, when the heart experiences irregularities in its coordinated contraction, due to electrophysiologic disturbances caused by a disease process or from an electrical disturbance, the heart is denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event.
Cardiac arrhythmias occurring in the atrial region of the heart are called supraventricular tachyarrhythmias (SVTs). Cardiac arrhythmias occurring in the ventricular region of the heart are called ventricular tachyarrhythmias (VTs). SVTs and VTs are morphologically and physiologically distinct events. VTs take many forms, including ventricular fibrillation and ventricular tachycardia. Ventricular fibrillation is a condition denoted by extremely rapid, nonsynchronous contractions of the ventricles. This condition is fatal unless the heart is returned to sinus rhythm within a few minutes. Ventricular tachycardia are conditions denoted by a rapid heart beat, 150 to 250 beats per minute, that has its origin in some abnormal location within the ventricular myocardium. The abnormal location is typically results from damage to the ventricular myocardium from a myocardial infarction. Ventricular tachycardia can quickly degenerate into ventricular fibrillation.
SVTs also take many forms, including atrial fibrillation and atrial flutter. Both conditions are characterized by rapid uncoordinated contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria can also adversely effect the ventricular rate. This occurs when the aberrant contractile impulse in the atria are transmitted to the ventricles. It is then possible for the aberrant atrial signals to induce VTs, such as a ventricular tachycardia.
Implantable cardioverter/defibrillators (ICDs) have been established as an effective treatment for patients with serious ventricular tachyarrhythmias. The first generation of ICDs relied exclusively on ventricular rate sensing for tachyarrhythmia detection. Specificity to SVT was, however, often compromised, especially when the ventricular response to SVT surpassed the patient""s heart rate during VT. The frequency of inappropriate shocks with early generations of signal chamber ICDs ranged from 10-41% of the shocks. Detection enhancements, such as Sudden Onset and Stability of the cardiac rhythms, improved specificity in more modem ICDs. The introduction of dual chamber defibrillators further improved upon the specificity to SVT without compromising sensitivity to VT. unfortunately, some patients still receive inappropriate therapies for SVT, especially when atria-to-ventricular conduction is 1:1.
Morphology-based algorithms have been proposed as a way of distinguishing VT from SVT. Many of these algorithms are template matching algorithms which determine the type of tachycardia by comparing features of the electrogram in question with an efficient representation of the patient""s normal sinus rhythm (NSR) electrogram. The basis of appropriate discrimination using template-matching algorithms are based on the assumption that the morphology of ventricular depolarization during VT will be dissimilar to those during NSR. These algorithms classify cardiac complexes based on their morphological similarity to the patient""s NSR complexes using only one intracardiac electrogram channel. In the process of comparing any two complexes, the algorithm locates a fuducial point (e.g., the peak of the complex) to align the two complexes with respect to each other. This alignment has the side effect of positioning complexes such that they appear to be more similar then they actually are. As a result, differentiating the two complexes becomes more difficult.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for providing a reliable system of discriminating SVT induced ventricular tachycardia from malignant ventricular tachycardia which can provide effective and reliable therapy to patients experiencing malignant ventricular tachycardia.
The present subject matter is directed to a system and a method for distinguishing between the occurrence of a ventricular tachycardia (VT) and a supraventricular tachycardia (SVT) during a tachycardia episode. Upon detecting a tachycardia episode, the system measures voltage values from predetermined positions along sensed cardiac signals. The voltage values are then compared to voltage values measured at the same relative positions along model cardiac complexes. Using this comparison, the system is able to distinguish the underlying cause of a tachycardia episode as either being an SVT or as a VT.
Initially, a first model cardiac complex and a second model cardiac complex are detected, or sensed, in the first cardiac signal and the second cardiac signal, respectively. In one embodiment, the first and second model cardiac complexes are normal sinus rhythm (NSR) cardiac complexes sensed during normal sinus rhythm. Alternatively, the first and second model cardiac complexes are cardiac complexes which are induced by electrical pulses delivered to at least one supraventricular location of the heart.
As the second model cardiac complex is sensed, a predetermined alignment feature is identified. In one embodiment, the predetermined alignment feature of the second cardiac complex is a repeatably identifiable portion of the second cardiac complex, such as a maximum deflection point of the second cardiac complex. The predetermined alignment feature is then used to define, or position, a datum at a specified interval from the predetermined alignment feature. In one embodiment, the datum can be thought of as a line, or a position, from which to make voltage measurements along the first cardiac signal during the first cardiac complex. In one embodiment, the datum is positioned at any location between two sensed cardiac complexes.
Once the datum has been positioned relative the predetermined alignment feature, a specified interval is measured between the predetermined alignment feature and the datum. Two or more morphological features are then selected along the first model cardiac complex. A measurement interval is then measured between the datum and each of the morphological features on the first model cardiac complex. In addition to measuring the measurement intervals, the voltage value of the first model cardiac complex at each of the measurement intervals is measured from the first model cardiac signal. The values and locations of the predetermined alignment feature, the specified interval and the measurement intervals are then recorded and stored for use in classifying cardiac complexes as either VT or SVT cardiac complexes during a tachycardia episode.
When a tachycardia episode is detected, a first cardiac complex and a second cardiac complex of a cardiac cycle are detected, or sensed, in the first cardiac signal and the second cardiac signal, respectively. As the second cardiac complex is sensed, the predetermined alignment feature is identified. The predetermined alignment feature is then used to define, or position, the datum at the specified interval from the predetermined alignment feature. Once the datum has been positioned relative the predetermined alignment feature, a voltage value is measured at each of the two or more measurement intervals from the datum along the first cardiac signal. The voltage values measured from the first cardiac signal are then compared voltage values measured from model complexes. Based on the comparison, the first cardiac complex is classified as either a VT complex or a SVT complex.
As the tachycardiac episode occurs, a plurality of cardiac cycles are detected in the first cardiac signal and the second cardiac signal. A predetermined number of the first cardiac complexes are classified as either VT or SVT cardiac complexes based on the present subject matter. A ventricular tachycardia episode is then declared when a threshold number of the predetermined number of the first cardiac complexes are classified as ventricular tachycardia complexes. Alternatively, a supraventricular tachycardia episode is declared when the threshold number of the predetermined number of the first cardiac complexes are classified as supraventricular tachycardia complexes.
In an additional embodiment, the present subject matter provides for a system and method of creating a template for a morphology-based algorithm which is used to classify cardiac complexes during a tachycardia episode. In one embodiment, electrical energy pulses are provided to the supraventricular region of the patient""s heart. The resulting cardiac complexes are then sensed and used to create a template for use in a morphology-based cardiac classification algorithm for classifying, categorizing or assessing a patient""s cardiac condition.